linux-kernel - Re: [PATCH 0/9] xen/x86: PVH Dom0 fixes and fallout adjustments

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Message-ID: <YUDNh9r8nYOxB76n@MacBook-Air-de-Roger.local>
Date:   Tue, 14 Sep 2021 18:27:51 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Jan Beulich <jbeulich@...e.com>
CC:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 0/9] xen/x86: PVH Dom0 fixes and fallout adjustments

On Tue, Sep 14, 2021 at 05:13:52PM +0200, Jan Beulich wrote:
> On 14.09.2021 14:41, Roger Pau Monné wrote:
> > On Tue, Sep 14, 2021 at 01:58:29PM +0200, Jan Beulich wrote:
> >> On 14.09.2021 13:15, Roger Pau Monné wrote:
> >>> On Tue, Sep 14, 2021 at 11:03:23AM +0200, Jan Beulich wrote:
> >>>> On 14.09.2021 10:32, Roger Pau Monné wrote:
> >>>>> On Tue, Sep 07, 2021 at 12:04:34PM +0200, Jan Beulich wrote:
> >>>>>> 2) Dom0, unlike in the PV case, cannot access the screen (to use as a
> >>>>>>    console) when in a non-default mode (i.e. not 80x25 text), as the
> >>>>>>    necessary information (in particular about VESA-bases LFB modes) is
> >>>>>>    not communicated. On the hypervisor side this looks like deliberate
> >>>>>>    behavior, but it is unclear to me what the intentions were towards
> >>>>>>    an alternative model. (X may be able to access the screen depending
> >>>>>>    on whether it has a suitable driver besides the presently unusable
> >>>>>>    /dev/fb<N> based one.)
> >>>>>
> >>>>> I had to admit most of my boxes are headless servers, albeit I have
> >>>>> one NUC I can use to test gfx stuff, so I don't really use gfx output
> >>>>> with Xen.
> >>>>>
> >>>>> As I understand such information is fetched from the BIOS and passed
> >>>>> into Xen, which should then hand it over to the dom0 kernel?
> >>>>
> >>>> That's how PV Dom0 learns of the information, yes. See
> >>>> fill_console_start_info(). (I'm in the process of eliminating the
> >>>> need for some of the "fetch from BIOS" in Xen right now, but that's
> >>>> not going to get us as far as being able to delete that code, no
> >>>> matter how much in particular Andrew would like that to happen.)
> >>>>
> >>>>> I guess the only way for Linux dom0 kernel to fetch that information
> >>>>> would be to emulate the BIOS or drop into realmode and issue the BIOS
> >>>>> calls?
> >>>>
> >>>> Native Linux gets this information passed from the boot loader, I think
> >>>> (except in the EFI case, as per below).
> >>>>
> >>>>> Is that an issue on UEFI also, or there dom0 can fetch the framebuffer
> >>>>> info using the PV EFI interface?
> >>>>
> >>>> There it's EFI boot services functions which can be invoked before
> >>>> leaving boot services (in the native case). Aiui the PVH entry point
> >>>> lives logically past any EFI boot services interaction, and hence
> >>>> using them is not an option (if there was EFI firmware present in Dom0
> >>>> in the first place, which I consider difficult all by itself - this
> >>>> can't be the physical system's firmware, but I also don't see where
> >>>> virtual firmware would be taken from).
> >>>>
> >>>> There is no PV EFI interface to obtain video information. With the
> >>>> needed information getting passed via start_info, PV has no need for
> >>>> such, and I would be hesitant to add a fundamentally redundant
> >>>> interface for PVH. The more that the information needed isn't EFI-
> >>>> specific at all.
> >>>
> >>> I think our only option is to expand the HVM start info information to
> >>> convey that data from Xen into dom0.
> >>
> >> PHV doesn't use the ordinary start_info, does it?
> > 
> > No, it's HVM start info as described in:
> > 
> > xen/include/public/arch-x86/hvm/start_info.h
> > 
> > We have already extended it once to add a memory map, we could extend
> > it another time to add the video information.
> 
> Okay, I'll try to make a(nother) patch along these lines. Since there's
> a DomU counterpart in PV's start_info - where does that information get
> passed for PVH? (I'm mainly wondering whether there's another approach
> to consider.)

We don't pass the video information at all for PVH, neither in domU or
dom0 modes if that's what you mean. Not sure what video information we
could pass for domU anyway, as that would be a PV framebuffer that
would need setup ATM. Maybe we could at some point provide some kind
of emulated or passed through card.

The information contained in start_info that's needed for PVH is
passed using hvm params, just like it's done for plain HVM guests. We
could pass the video information in a hvm param I guess, but it would
require stealing guest memory to store it (and mark as reserved in
the memory map). Not sure that's better than expanding HVM start info.

Maybe there's another hypercall interface I'm missing we could use to
propagate that information to dom0?

Thanks, Roger.